A recent development in communications technology has been the introduction of asynchronous transfer mode (ATM) networks. Such networks are connectionless in nature and transport traffic in cells which are routed over virtual channel connections. These broadband networks, which were originally developed for transporting data traffic, are now being used for the transport of narrow band voice traffic. It will of course be understood that voice transport presents particular problems as the traffic must be carried end to end in real time and with a guaranteed quality of service. A number of techniques have been developed to address these issues, for example the introduction of ATM Adaptation Layer Two (AAL2) in which voice traffic from a number of users is multiplexed into minicells transported in ATM cells. These techniques will be familiar to those skilled in the art.
A further problem with carrying voice traffic over an asynchronous network is that of setting up calls in response to user demand. When a call is set up, network resources must be allocated and reserved for that call to ensure that the guaranteed quality of service can be supported.
In an ATM network, calls are established between service endpoints over ATM virtual channel connections (VCCs). Individual VCCs may support calls by means of ATM Adaptation Layer Type One (AAL1), ATM Adaptation Layer Type Two (AAL2), ATM Adaptation Layer Type Three/Four (AAL3/4) or ATM Adaptation Layer Type Five (AAL5). In particular, the use of AAL2 is advantageous in situations where voice information is compressed/decompressed at the service endpoints. This results in short packets of information per voice call such that multiple voice calls can be multiplexed on to a single AAL2 VCC with concomitant savings in bandwidth.
It will be appreciated that, before a connection request can be admitted to the network, an appropriate channel must first be allocated to the request. This is typically performed by a negotiation procedure between an originating and a terminating end point. Under certain conditions, it is possible for a channel selected for a call by one end point to be unacceptable to the other. Typically such a situation occurs when the particular channel that has been chosen does not terminate on an appropriate resource at the terminating endpoint that would allow the call request to be satisfactorily completed. In such circumstances the call set up procedure will have to be aborted, even if there exist other usable channels that terminate on a resource that would have permitted the call to be completed. This situation is defined as premature blocking. The volume of aborted call set up attempts resulting from premature blocking can represent a significant waste of the network resources involved in the failed set up attempts as well as a loss of revenue to a network operator.